This invention relates to a differential for a vehicle cross drive, and more particularly to a compact and simplified planetary steering differential.
A large number of track laying work machines have been developed which have incorporated either clutch-and-brake steering mechanisms or geared steering mechanisms. In both of these examples, however, the drive to the opposite tracks is discontinuous when making turns. This results in poorer operation of the work machine than is desired, especially during turns in marginal ground conditions.
In some seasons of the year, for example, logging industry track laying work machines with conventional clutch-and-brake steering mechanisms cannot be maneuvered to provide useful work because when one of the steering clutches is disengaged the power to that side of the tractor is interrupted. The other side of the tractor then has to pull the total drawbar load to maintain momentum. Unfortunately, soil strength under only one side is often inadequate to carry the total load so the powered track shears the soil and loses traction. Such a loss is a particular disadvantage when it comes to making relatively small steering corrections. On the other hand, clutch-and-brake steering mechanisms are widely used because they perform well in straight-ahead work applications and are simple in construction.
In geared steering mechanisms, rather than disconnecting one track and/or bringing that track to rest, one track is driven at a lower speed than the other by having additional gear sets in the drive to each track. But these mechanisms are complex and costly in construction because duplicate planetary sets, gears, brakes and/or clutches are typically provided at both sides. Furthermore, since these mechanisms are discontinuous a lower mean track speed is provided during a turn.
Another major group of steering mechanisms includes differential mechanisms in which drive is transmitted continuously to both tracks. The simplest form thereof is a braked differential, but these are rarely used because of relatively large power losses at the steering brake. Many of the disadvantages of the braked differential are obviated by controlled differentials. In such mechanisms engine power is not wasted in the steering brakes since power is merely transferred from the inner track to the outer track. But these also have disadvantages. One major disadvantage is that when the brakes are off these mechanisms act as simple differentials so that they depend on the reaction between the track and the ground to be equal for straight-ahead operation. Since this is often not true the work machine tends to drift so that frequent corrections are required. Moreover, if one track loses traction the drive to the opposite track is reduced.
One or more of the above mentioned problems can be overcome by the use of double differentials and equivalent mechanisms which usually can counter rotate the opposite output members for spot turns. Typically, two differentials or their equivalent are arranged in parallel with their output shafts interconnected by gearing. Usually, a main drive power path is provided to one of the differentials and a steering drive power path is provided to the other one of the differentials. In some instances the steering input shaft has been driven by a hydrostatic pump and motor system, with the ability to hold the steering input shaft stationary to prevent differential action and to assure straight ahead operation of the work machine. One major deficiency thereof is that dual cross shafts and associated gearing are required so that the construction is not only complex and costly, but also an unnecessarily large housing is required for containment of the components.
The present invention is directed to overcoming one or more of the problems as set forth above.
A planetary steering differential adapted to be driven in use by a transmission and a steering motor. The planetary steering differential includes a first input arrangement adapted to be driven by the transmission, a second input arrangement adapted to be driven by the steering motor and first and second output members. First and second gear trains are provided for rotating the first and second output members in the same direction and at the same speed in response to rotation solely of said first input arrangement and by holding the second input arrangement stationary. The first and second gear trains cause the speed of the output members to be reduced from the speed of the first input arrangement. The output members are caused to rotate in opposite directions at the same speed in response to rotation solely of the second input arrangement and holding the first input member stationary. The first and second gear trains include first and second interconnected planetary reduction arrangements respectively. The first input arrangement is connected to the first planetary reduction arrangement and the second input arrangement is to the first planetary reduction arrangement. The first and second gear trains are arranged on and interconnected solely along a common central axis.